The aetiology of Common Variable Immunodeficiency (CVID) has fascinated immunologists since Dr. Janeway reported the first case in 1953. While the advances in molecular biology have enlightened us on the aetiology in some patients, the majority is not caused by inherited genetic disorders. A convincing mechanism accounting for the intrinsic variable and partial nature of the condition has yet been proposed. CVID separates itself from other primary antibody deficiencies by the procurement of an abnormal T-cell compartment. Data from this study support that both T-cells and B-cells are subjected to similar deficiency. Investigation of the T-cell receptor repertoire by next-generation sequencing and multi-parametric flow cytometry suggests a severe reduction in naïve T-cell output from the thymus. Similarly, the study of long-lived plasma cell generation and survival highlighted the greatest functional deficits in the naïve B-cell pool, altogether supporting an acquired arrest in lymphogenesis. Using DNA methylation as a surrogate marker for pre-VDJ clonality, this study further shows that some CVID patients exhibited clonal haematopoiesis, adjoining CVID to other clonal haematopoiesis related acquired haematological disorders. Further work is being focused on using high resolution techniques to confirm this association and mechanistically define the development of antibody deficiency in adulthood.